1. Field of the Invention
The present invention relates generally to a method of producing a light conductive plate type light source. More particularly, the present invention relates to a method of producing a light conductive plate type light source preferably employable as a back light source for a liquid crystal .display unit which is widely used as a display section for a television, an office automation equipment or the like at present.
2. Background Art
To facilitate understanding of the present invention, a conventional method of producing a light conductive plate type light source of the foregoing type will be described below with reference to FIG. 4.
FIG. 4 shows by way of fragmentary enlarged sectional view the structure of a light conductive plate type light source 90 produced by employing the conventional method. As is apparent from the drawing, the light conductive plate type light source 90 includes a light conductive plate 91 which is formed of a high transparent material such as an acrylic resin, and at least one sheet-shaped light diffusing plate 92 is secured to the light conductive plate 91 on a front surface 91a side while coming in substantially close contact with the light conductive plate 91 so as to allow a light beam to be emitted from the front surface 91a of the light conductive plate 91 with excellent uniformity, while a sheet-shaped light reflecting plate 93 is secured to the light conductive plate 91 on a rear surface 91b side of the light conductive plate 91 in order to improve a lighting efficiency of the light conductive plate type light source 90.
The light conductive plate 91 and the light diffusing plate 92 (or the light reflecting plate 93) are adhesively secured to each other with double-sided adhesive tapes 94 interposed therebetween. In the case that a plurality of light diffusing plates 92 are used for the light conductive plate type light source 90 in order to improve a light diffusing property of the latter, the same securing operation to be achieved using double-sided adhesive tapes 94 as mentioned above is repeated to build a laminated structure with the light diffusing plates 92.
Next, another conventional method of producing a light conductive plate type light source will be described below with reference to FIG. 5.
FIG. 5 shows by way of fragmentary enlarged sectional view the state that three light diffusing plates 92 are placed on a light conductive plate 91 in the overlapped state at a predetermined position relative to the light conductive plate 91, and while this overlapped state is maintained, both the light conductive plate 91 and the light dispersing plates 92 are welded together by operating a supersonic welding machine 80 from the light diffusing plate 92 side without any use of double-sided adhesive tape. At this time, in the case that a plurality of light diffusing plates 92 are used for the light conductive plate type light source 90, there rises an occasion that a welding operation is completed merely by way of a single step. In practice, since it is very difficult that a plurality of light diffusing plates 92 are welded together without any fluctuation in strength, the light diffusing plates 92 are usually welded together one by one in such a manner that a subsequent light diffusing plate 92 is welded to the preceding light diffusing plate which has been already welded to the underlying light diffusing plate.
However, the above-described conventional methods have the following problems. Since both-sided adhesive tapes 94 are used for achieving each securing operation, an extra quantity of adhesive is liable of flowing outside of the laminated structure. For this reason, it is required that each securing operation is carefully performed. This leads to a problem that a securing operation is achieved for the light conductive plate type light source 90 at an increased cost. Other problem is that a gap substantially equal to a thickness of the double-sided adhesive tape 94 arises between the light conductive plate 91 and the light diffusing plate 92, causing the light diffusing plate 92 to be concavely warped, resulting in a quality of appearance of the light conductive plate type light source 90 being degraded. Another problem is that when a repairing operation is required after completion of a incorrect securing operation, an adhesive remaining on the double-sided adhesive tape 94 should be removed from the light conductive plate 91 while consuming a long time and many manhours.
In the case that a supersonic welding machine is employed for achieving each securing operation, it is necessary that a material constituting the light conductive plate 91 and a material constituting the light diffusing plate 92 can be welded to each other. Since a polyethylene terephthalate resin having an excellent light diffusing property, i.e., a typical material, employable for the light diffusing plate 92, can not be welded to the material constituting the light conductive plate 91, there arises a problem that the kinds of materials employable for the light conductive plate, the light diffusing plate 92, and the light reflecting plate are limitatively determined. Once the light conductive plate 91 and the light diffusing plate 92 are welded to each other, it is impossible to perform a repairing operation for the light conductive plate type light source 90. In addition, as the number of light diffusing plates 92 increases, manhours required for achieving a securing operation proportionally increases regardless of the type of method employed for weldably securing both the light conductive plate 91 and the light scattering plates 92 together. Thus, there arises a problem that the light conductive plate type light source 90 having a plurality of light diffusing plates is produced at an increased cost.